This invention relates to analytical systems and processes, and more particularly to systems and processes for the analysis of materials for mercury content.
Mercury is generally found as ionic or organic mercury in nature. Examples of ionic mercury include inorganic compounds in rocks and mercury salts in water. When a strong acid is added to such materials, the products can be made water soluble with the resulting mercury cations being completely dissociated from the counter group anions. Analysis of mercury in the cation state may be performed by the cold vapor (room temperature) method which involves adding a strong reducing agent such as stannous chloride to the sample mixture to convert mercury cations to elemental mercury which is insoluble in aqueous solution. The resulting elemental mercury vaporizes in air in a manner similar to water evaporating, and the vaporized mercury may be analyzed in an atomic absorption system by passing the vaporized mercury through a beam of light with a spectral wavelength of 253.7 nanometers.
In environmental pollution investigations, mercury is most often found as part of an organic compound in forms such as phenyl mercury or methyl mercury. Organo-substituted mercury compounds must be treated by a more vigorous procedure than described above that includes the steps of digestion by a strong acid or base and then breaking apart with a strong oxidizer such as permanganate, bromate or vanadium pentoxide, with the subsequent elimination of excess oxidant by the addition of hydroxyl ammonium chloride. Among aspects that complicate mercury assays are the facts that nearly everything contains some trace level of mercury, and that compounds other than mercury can absorb radiation at 253.7 nanometers and give a "false positive" for mercury.
An EPA Protocol describes a manual mercury analysis by cold vapor technique that involves placing a sample aliquot diluted to 100 mL in a 300 mL bottle; adding reagents including 5 mL of stannous sulfate; immediately attaching the bottle to aeration apparatus; aerating the bottle with a circulating pump; and passing the resulting vapor through a desiccant chamber and an absorption cell to a scrubber.
In accordance with one aspect of the invention, there is provided an automated cold vapor mercury assay system that includes a sample inlet, a storage chamber of predetermined volume for storing a sample of material to be analyzed, a reaction chamber having first and second inlet ports and an outlet port, a reducing agent reservoir, a transport liquid reservoir, distributor structure coupled to the sample storage chamber and movable between a first position connecting the storage chamber to the sample inlet and a second position connecting the storage chamber to the reaction chamber, analyzer apparatus coupled to the outlet port of the reaction chamber structure, and controller structure. The controller in an analysis sequence places the distributor in the first position for flowing sample to be analyzed into the storage chamber structure from the sample inlet while concurrently flowing a reducing agent from the reducing agent reservoir structure into the reaction chamber structure; then supplies a source of gas for flow through the reaction chamber to release residual elemental mercury from the reducing. agent; then moves the distributor to the second position and applies transport liquid through the distributor to the storage chamber for flowing the sample to be analyzed into the reaction chamber through the second inlet port; and again applies agitating gas to agitate the material in the reaction chamber to form elemental mercury vapor for transport to the analyzer apparatus for analysis.
In accordance with another aspect of the invention, there is provided an automated cold vapor mercury assay system that includes a sample inlet, a storage chamber of predetermined volume for storing a sample of material to be analyzed, a reaction chamber having first and second inlet ports and an outlet port, a reducing agent reservoir, a transport liquid reservoir, distributor structure coupled to the sample storage chamber and movable between a first position connecting the storage chamber to the sample inlet and a second position connecting the storage chamber to the reaction chamber, analyzer apparatus coupled to the outlet port of the reaction chamber, and conduit structure coupled between the analyzer apparatus and the first reaction chamber inlet port of said for obtaining a reference value of agitating gas and then applying the agitating gas to material in the reaction chamber structure.
In accordance with another aspect of the invention, there is provided a cold vapor mercury assay system that includes a sample inlet, a storage chamber of predetermined volume for storing a sample of material to be analyzed, a reaction chamber having first and second inlet ports and an outlet port, and analyzer apparatus coupled to the outlet port of the reaction chamber. The reaction chamber includes a body portion of cylindrical configuration in which a sample inlet is coupled for tangential introduction of sample material to be assayed; a base portion of hemispherical configuration in which a reagent inlet port is formed, and an upper portion of conical configuration to which an outlet port is coupled, the conical upper portion having an inner surface of concave configuration.
In preferred embodiments, the storage chamber has a volume of less than ten milliliters, and the reaction chamber has a volume of less than fifty milliliters but greater than that of the storage chamber. Dryer material is coupled in circuit between the reaction chamber outlet port and the analyzer apparatus. The agitating gas is preferably selected from the group consisting of nitrogen, helium, argon, and air and has a purity of at least 0.998.
In a particular embodiment, the analyzer is of the atomic absorption type and includes a light source for generating a beam of light with a spectral wavelength of 253.7 nanometers; first and second photosensors; and an analysis cell with two parallel passages disposed between the radiation source and the radiation sensors, and each passage has an aspect ratio (length/diameter) of at least one hundred. The body portion of the reaction chamber has a diameter of less than five centimeters and a length in the range of three to eight centimeters; the base portion is hemispherical and has a radius of less than three centimeters and a first inlet port is disposed at the bottom of the base portion; and the upper portion has a length in the range of three to six centimeters and a concave inner surface that tapers smoothly from said body portion to said outlet port at the top of said upper portion. Autosampler structure is coupled to a first port of the distributor and reduced pressure applying apparatus is coupled to a second port of the distributor for drawing a sample from the autosampler into the storage chamber when the distributor is in its first position.
The small internal volumes in the particular embodiment enable the system to measure very low levels of mercury (one part per trillion). Minimal volumes are provided throughout the system. The largest volume in the system is the reaction chamber, and the concave shape of upper portion of that chamber contributes to the small internal volume. That system has a detection limit of less than one part per trillion.